When considering the recovering of the seats, it is wise to consult with the person who maintains you aircraft first. Though it is an excellent idea, it is technically "maintenance", and as such, should be expected to conform to the required design (flammability), and be signed out. I very highly recommend leather. It is more expensive, but will last much longer, can be cleaned much more easily and completely (barf), and is assured to meet the flammability requirements, should anyone ask.
Be sure that when the old seat coverings are removed, the seat frame is completely disassembled, paint stripped, and inspected by the maintenance outfit. cracking of these frames is quite common. The new coverings will probably be glued on, and there is no point in spending the money to glue on new coverings, to have to take it all back off again next inspection, because an unseen crack opened up! There is also an AD on these seats now, excellent time to perform that inspection too!
There are several sources of interior plastic parts. Airtex is the one I know, though I have heard of others. Be sure to get the plastic for the right model of the 150, as there were fit changes over the years. Plan to do some trimming, and hole locating during installation.